Conformable copolyester film

ABSTRACT

A conformable oriented polymeric film comprising a layer of copolyester derived from: (i) one or more diol(s); (ii) an aromatic dicarboxylic acid; and (iii) one or more aliphatic dicarboxylic acid(s) of the general formula C n H 2n (COOH) 2  wherein n is 2 to 8, wherein the aromatic dicarboxylic acid is present in the copolyester in an amount of from 60 to 85 mole % based on the total amount of dicarboxylic acid components in the copolyester, wherein the copolyester is a random or alternating copolyester, and wherein after deformation by subjecting and maintaining the film to 7% strain, the stress within the film immediately after deformation dissipates to a residual value of less than 60% of the original value within 10 minutes of said deformation; and use thereof as a conformable medium for signage, advertising, graphic art and other display applications or as an overlaminate for such media.

[0001] This invention relates to a conformable copolyester film, and itsuse as a medium for signage, advertising, graphic art and other displayapplications or as an overlaminate for such media.

[0002] Plastic film is used extensively as a medium for signage,advertising, graphic art and other forms of visual communication.Examples of this include labels, stickers, banners, vehicle livery,decal and posters. Clear plastic film is also used for overlamination,either as an integral part of a multilayer construction or as anadditional coating component on an existing image or message. Filmswhich provide a “no-label” look, i.e. films which appear to besubstantially indistinguishable from the surface to which the film isapplied, are particularly desirable. Other visual appearances, includingwhite, silver, gloss and matte are also well-known in the art.

[0003] The range of use is in practice very large and consequently therequired performance of film can vary greatly. Thus a number ofdifferent types of plastic can be employed to form the plastic film andtherefore provide the diversity of properties. Nevertheless a marketsurvey of the graphic arts industry in Europe (Alexander WatsonAssociates Publication: European Annual Review, 1998; “Self-AdhesiveLabelstock and Graphic Arts”) reported that film made from plasticisedpolyvinyl chloride (p-PVC) accounts for approximately 80% of allprinting substrate consumed.

[0004] Polyvinyl chloride films (PVC) have been used extensively becausesuch films are highly flexible, particularly when they contain additivessuch as plasticizers and/or tackifiers. These flexible films areparticularly suitable for graphic works intended to be adherable toirregular or curved surfaces, including for instance surfaces atright-angles to each other, without the need for strong adhesive,enabling the graphic work to be repositioned or removed from the surfaceand, if desired, re-used on a different surface. Generally, PVC filmscan be calendered films or cast films. The films generally comprise 45%to 50% by weight of PVC resin together with stabilizers, pigment and aplasticiser such as dioctyl phthalate (DOP). The thickness of such filmsranges from about 0.01 to about 0.25 mm. The film is generally suppliedwith a release liner which is usually paper or paperboard stock eitherwith a cast-coated or poly-coated surface on one side. Thecharacteristic of plasticised PVC film which provides the flexiblenature is its ability to dissipate or relax internal stress afterdeformation. When a film of p-PVC is printed and attached to asupporting surface using an appropriate adhering means, the materialwill deform easily and adopt the surface topography of the support,providing a pleasing appearance to the display. Due to the material'sstress-dissipation properties, very little stress remains afterapplication to the surface to cause the film to recover its originalsize and shape. Such films do not therefore tend to “lift” away from theunderlying surface.

[0005] One method to assess the ability of film to dissipate stresscomprises testing a strip of film in an extensometer. Typically aspecimen of film is strained by 7% (0.07 strain) and held at these newdimensions for a period of time. Plasticised PVC film of the typeavailable commercially displays a stress of around 2.3 kg/mm²immediately after deformation but this will fall at room temperature byabout 80% to around 0.4 kg/mm² within about 10 minutes after stretching.

[0006] The plasticisation of PVC has two primary effects which conferthe advantageous properties mentioned above. Firstly, the glasstransition temperature (Tg) is lowered and broadened such that thematerial becomes compliant (reduces in stiffness) and easily deformed atambient temperature. Secondly, the viscous character dominates theelastic nature of the material, to enable the “relaxation” mechanism tooccur.

[0007] There are, however, various disadvantages associated with thecurrent use of plasticised PVC films. The decomposition or incinerationof p-PVC products may result in the release of dioxins and hydrogenchloride, which is environmentally undesirable. In addition, theplasticiser component (e.g. phthalate esters such as dioctyl phthalate)has been linked with various physiological side-effects. Furthermore,plasticised PVC films suffer from relatively poor solvent resistance,weatherability and tear-resistance. In addition, p-PVC films oftenexhibit undesirable shrinkage when printed, for example when usingink-jet techniques. It would be desirable to provide a replacement forplasticised PVC films which does not suffer from such disavantages.

[0008] Polyester films have long been used in display applicationsincluding labelling. However, because of their stiffness and highdimensional stability, they have not generally been preferred forapplications in which the film is required to conform closely to theshape of the object to which the film or label is applied (B. L.Kindberg and R. M. Kimmel, “films: Flexibility in Labelling”, in Paper,Film and Foil Converter, April 1981). Polyester films have generallytherefore only been used in labelling intended for relatively flatsurfaces or in conjunction with strong adhesives.

[0009] It is, however, known to lower the Tg of polyester films, andthereby increase flexibility, by incorporating additional co-monomers.However, in polyesters which are designed to possess a low Tg and lowmodulus (i.e. low stiffness), it is common for elastic behaviour todominate over viscous behaviour, i.e. polyesters having a sufficientlylow Tg to exhibit good flexibility generally also exhibit a significantdegree of elastic character, which is not desirable in the applicationsmentioned above due to the inherent “shape memory” or elasticity. It hashitherto been difficult to achieve the combined properties required forthe applications mentioned above.

[0010] It is an object of this invention to provide an alternativeconformable film suitable as a medium for display applications whichdoes not suffer from one or more of the disadvantages mentioned above.In particular, it is an object of this invention to provide a film whichnot only exhibits good flexibility and is adherable to irregular orcurved surfaces, including surfaces at right-angles to each other, butalso avoids the environmental and/or weatherability and/or tearresistance and/or solvent resistance and/or shrinkage problems ofexisting p-PVC films. It is a further object of this invention toprovide a film which not only exhibits good flexibility and is adherableto irregular or curved surfaces, including surfaces at right-angles toeach other, without the need for strong adhesive, enabling the film tobe repositioned on, or removed from, the object to which it has beenapplied and, if desired, re-used on a different object, but also avoidsthe environmental and/or weatherability and/or tear resistance and/orsolvent resistance and/or shrinkage problems of existing p-PVC films.

[0011] According to the present invention, there is provided aconformable oriented polymeric film comprising a layer of copolyesterderived from:

[0012] (i) one or more diol(s);

[0013] (ii) an aromatic dicarboxylic acid; and

[0014] (iii) one or more aliphatic dicarboxylic acid(s) of the generalformula C_(n)H_(2n)(COOH)₂ wherein n is 2 to 8,

[0015] wherein the aromatic dicarboxylic acid is present in thecopolyester in an amount of from 60 to 85 mole % based on the totalamount of dicarboxylic acid components in the copolyester, wherein thecopolyester is a random or alternating copolyester, and wherein afterdeformation by subjecting and maintaining the film to 7% strain, thestress within the film immediately after deformation dissipates to aresidual value of less than 60% of the original value within 10 minutesof said deformation.

[0016] The inventors of the present Application have found unexpectedlythat the copolyesters defined above not only exhibit a suitably low Tg(around room temperature), and thereby improve the compliance of thefilm, but also exhibit improved viscous or relaxation properties.Moreover, rather than existing as viscous fluids, the copolyesters canbe processed into a film, biaxially drawn and crystallised (heat-set)using conventional methods for polyester film processing. Thus, the filmexhibits good flexibility and is adherable to irregular or curvedsurfaces, including surfaces at right-angles to each other, without theneed for strong adhesive, enabling the film to be repositioned on, orremoved from, the object to which it has been applied and, if desired,re-used on a different object. The film of the present invention istherefore particularly useful as a conformable film suitable as a mediumfor signage, advertising, graphic art and other display applications oras an overlaminate for such media. Where used as an overlaminate, thefilm may be used to protect existing signage, advertising and graphicart, for instance to provide scratch resistance and/or UV-protection.The film does not have the environmental drawbacks of existing p-PVCconformable films noted above. In addition, the film exhibits improvedtear resistance and shrinkage properties in relation to existing P-PVCfilms, as well as improved weatherability and solvent resistance.

[0017] The term “conformable film” as used herein means a film whichwill readily deform and adopt the surface topography of the object towhich it is applied without recovering its original size and shape andwithout lifting away from the surface of the object. In one embodiment,the term “conformable film” means a film having stress relaxationproperties such that, after deformation by subjecting and maintainingthe film sample to 7% strain at −° C., the stress immediately afterinitial deformation dissipates to a residual value of less than 60%,preferably less than 50%, and most preferably less than 40% of theoriginal level within 10 minutes of said deformation. In addition to theproperty of stress dissipation, it is important that the initial andresidual stresses generated in the material as a result of permanentdeformation should be of a suitably low magnitude such that therestraining or frictional force of an adhesive is sufficient to fix thedeformed film in its new shape. Thus, for ease of application andsubsequent dimensional stability of the film, the initial stress in afilm sample after deformation by subjecting and maintaining the filmsample to 7% strain should preferably be less than 4.5 kg/mm², morepreferably less than 3.0 kg/mm², more preferably less than 2.0 kg/mm²,and most preferably less than 1.0 kg/mm². The residual stress within 10minutes of the said deformation should be preferably less than 1.7kg/mm², more preferably less than 1.3 kg/mm², more preferably less than0.9 kg/mm² and most preferably less than 0.4 kg/mm².

[0018] The thickness of the film is preferably from about 12 to about250 μm, more preferably from about 12 to about 150 μm, and typically isabout 12-100 μm in thickness. Where the film is used as an overlaminate,the film thickness is generally from about 12 to about 35 μm, typicallyabout 25 μm. Where the film is itself used as the signage/displaymedium, the film is typically about 20-100 μm, preferably about about50-60 μm, in thickness.

[0019] The copolyester film layer is a self-supporting film or sheet bywhich is meant a film or sheet which (i) is not a liquid or dispersion;(ii) is preferably capable of independent existence in the absence of asupporting base. In other words the film or sheet has an inherent form(i.e. film-like) before use in its intended application whichcorresponds to the form during use in its intended application. Thecopolyester is obtainable by condensing said dicarboxylic acids or theirlower alkyl (up to 6 carbon atoms) diesters with one or more diols. Thearomatic dicarboxylic acid is preferably selected from terephathalicacid, isophathalic acid, phthalic acid, 2,5-, 2,6- or2,7-naphthalenedicarboxylic acid, and is preferably terephthalic acid.The diol is preferably selected from aliphatic and cycloaliphaticglycols, e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol,neopentyl glycol and 1,4-cyclohexanedimethanol, preferably fromaliphatic glycols. Preferably the copolyester contains only one glycol,preferably ethylene glycol. The aliphatic dicarboxylic acid may besuccinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid,azeleic acid or sebacic acid. In one embodiment the aliphaticdicarboxylic acid is selected from succinic acid, adipic acid, azeleicacid and sebacic acid. Preferably the copolyester contains only onealiphatic dicarboxylic acid. Preferably the aliphatic dicarboxylic acidis adipic acid. Particularly preferred examples of copolyesters are (i)copolyesters of azeleic acid and terephthalic acid with ethylene glycol;(ii) copolyesters of adipic acid and terephthalic acid with ethyleneglycol; and (iii) copolyesters of sebacic acid and terephthalic acidwith an ethylene glycol. Particularly preferred copolyesters are thoseof adipic acid and terephthalic acid with ethylene glycol.

[0020] In a preferred embodiment the aromatic dicarboxylic acid ispresent in the copolyester in an amount of about 65 to about 75 mole %,preferably about 68 to about 72 mole %, typically at about 70 mole %based on the total amount of dicarboxylic acid component(s) in thecopolyester.

[0021] The copolyester used in the present invention is a random oralternating copolyester, as opposed to a block copolyester. Preferably,the copolyester is a random copolyester. Reference herein to a randomcopolyester means a copolyester wherein the different ester monomericunits, i.e. the [aromatic dicarboxylic acid-diol] units and the[aliphatic dicarboxylic acid-diol] units are situated randomly in thechain. Reference herein to an alternating copolyester means acopolyester wherein there is a definite ordered alternation of themonomeric ester units. Preference herein to a block copolyester means acopolyester wherein the chain consists of relatively long blocks of onetype of monomeric ester unit joined together followed by relatively longblocks of a different type of monomeric ester unit joined together.

[0022] Formation of the copolyester is conveniently effected in a knownmanner by condensation or ester interchange, generally at temperaturesup to about 275° C.

[0023] The glass transition temperature (Tg) of the copolyester ispreferably below about 50° C., more preferably below about 45° C., morepreferably below about 35° C., more preferably below about 30° C. andtypically about 25-30° C.

[0024] Preferably the film exhibits stress relaxation properties suchthat, after deformation by subjecting and maintaining the film sample to7% strain, the stress immediately after initial deformation dissipatesto a residual value of less than 60%, preferably less than 50%, and mostpreferably less than 40% of the original level within 10 minutes of saiddeformation.

[0025] Under these conditions of strain, the film should preferablyexhibit an absolute value of stress which, measured in the samedirection of strain, is initially less than 4.5 kg/mm², preferably lessthan 3.0 kg/mm², more preferably less than 2.0 kg/mm², and mostpreferably less than 1.0 kg/mm². The corresponding stress within 10minutes of the said deformation should be preferably less than 1.7kg/mm², more preferably less than 1.3 kg/mm², more preferably less than0.9 kg/mm² and most preferably less than 0.4 kg/mm². The film preferablyexhibits the properties of stress dissipation and low initial andresidual stress noted above in both the machine dimension (MD) and thetransverse dimension (TD).

[0026] The shrinkage of the film, measured as described herein, ispreferably less than 4%, more preferably less than 2%, and mostpreferably less than 1%, in either the machine dimension or thetransverse dimension. Methods of controlling shrinkage in the final filmby varying process parameters during the stretching and heat-settingsteps of film manufacture are well-known to the skilled person.

[0027] Formation of the film may be effected by conventional techniqueswell-known in the art. Conveniently, formation of the film is effectedby extrusion, in accordance with the procedure described below. Ingeneral terms the process comprises the steps of extruding a layer ofmolten polymer, quenching the extrudate and orienting the quenchedextrudate in at least one direction.

[0028] The film may be uniaxially-oriented, but is preferablybiaxially-oriented. Orientation may be effected by any process known inthe art for producing an oriented film, for example a tubular or flatfilm process. Biaxial orientation is effected by drawing in two mutuallyperpendicular directions in the plane of the film to achieve asatisfactory combination of mechanical and physical properties.

[0029] In a tubular process, simultaneous biaxial orientation may beeffected by extruding a thermoplastics polyester tube which issubsequently quenched, reheated and then expanded by internal gaspressure to induce transverse orientation, and withdrawn at a rate whichwill induce longitudinal orientation.

[0030] In the preferred flat film process, the film-forming copolyesteris extruded through a slot die and rapidly quenched upon a chilledcasting drum to ensure that the copolyester is quenched to the amorphousstate. Orientation is then effected by stretching the quenched extrudatein at least one direction at a temperature above the glass transitiontemperature of the copolyester. Sequential orientation may be effectedby stretching a flat, quenched extrudate firstly in one direction,usually the longitudinal direction, i.e. the forward direction throughthe film stretching machine, and then in the transverse direction.Forward stretching of the extrudate is conveniently effected over a setof rotating rolls or between two pairs of nip rolls, transversestretching then being effected in a stenter apparatus. Alternatively,the cast film may be stretched simultaneously in both the forward andtransverse directions in a biaxial stenter. Stretching is generallyeffected so that the dimension of the oriented film is from 2 to 5times, generally at least 2.5 times, preferably no more than 4.5 times,more preferably no more than 3.5 times its original dimension in the oreach direction of stretching. Typically, stretching is effected attemperatures in the range of 30 to 65° C., preferably in the range of 30to 50° C., and in any case higher than the Tg of the copolyester,preferably about 15° C. higher than the Tg of the copolyester. It is notnecessary to stretch equally in the machine and transverse directionsalthough this is preferred if balanced properties are desired.

[0031] A stretched film may be, and preferably is, dimensionallystabilised by heat-setting under dimensional restraint at a temperatureabove the glass transition temperature of the polyester but below themelting temperature thereof, to induce crystallisation of the polyester.The actual heat-set temperature and time will vary depending on thecomposition of the film but should not be selected so as tosubstantially degrade the tear resistant properties of the film.Moreover, if the heat-set temperature is too low, too littlecrystallinity may develop in the film and too little accompanyingamorphous chain relaxation will occur. This may reduce theconformability of the film, and in particular may undesirably increasethe initial and residual stress in the film after deformation. Withinthese constraints, a heat-set temperature of about 100 to 165° C.,preferably about 120 to 165° C., is generally desirable. Dimensionalrelaxation (“toe-in”), wherein the film is allowed to relax in a givendimension by up to about 5% and typically about 2-4% during theheat-setting step, may be used to modulate shrinkage of the film.

[0032] According to a further aspect of the invention, there is providedthe use of a conformable oriented polymeric film comprising a layer ofcopolyester derived from:

[0033] (i) one or more diol(s);

[0034] (ii) an aromatic dicarboxylic acid; and

[0035] (iii) one or more aliphatic dicarboxylic acid(s) of the generalformula C_(n)H_(2n)(COOH)₂ wherein n is 2 to 8,

[0036] wherein the aromatic dicarboxylic acid is present in thecopolyester in an amount of from 60 to 85 mole % based on the totalamount of dicarboxylic acid components in the copolyester, wherein thecopolyester is a random or alternating copolyester, and wherein afterdeformation by subjecting and maintaining the film to 7% strain, thestress within the film immediately after deformation dissipates to aresidual value of less than 60% of the original value within 10 minutesof said deformation, as or in the manufacture of a conformable mediumfor signage, advertising, graphic art and other display applications oras an overlaminate for such media.

[0037] According to a further aspect of the invention, there is providedthe use of a conformable oriented polymeric film comprising a layer ofcopolyester derived from:

[0038] (i) one or more diol(s);

[0039] (ii) an aromatic dicarboxylic acid; and

[0040] (iii) one or more aliphatic dicarboxylic acid(s) of the generalformula C_(n)H_(2n)(COOH)₂ wherein n is 2 to 8,

[0041] wherein the aromatic dicarboxylic acid is present in thecopolyester in an amount of from 60 to 85 mole % based on the totalamount of dicarboxylic acid components in the copolyester, wherein thecopolyester is a random or alternating copolyester, and wherein afterdeformation by subjecting and maintaining the film to 7% strain, thestress within the film immediately after deformation dissipates to aresidual value of less than 60% of the original value within 10 minutesof said deformation, as a conformable layer in a medium for signage,advertising, graphic art and other display applications or as anoverlaminate for such media.

[0042] According to a further aspect of the invention, there is providedthe use of a conformable oriented polymeric film comprising a layer ofcopolyester derived from:

[0043] (i) one or more diol(s);

[0044] (ii) an aromatic dicarboxylic acid; and

[0045] (iii) one or more aliphatic dicarboxylic acid(s) of the generalformula C_(n)H_(2n)(COOH)₂ wherein n is 2 to 8,

[0046] wherein the aromatic dicarboxylic acid is present in thecopolyester in an amount of from 60 to 85 mole % based on the totalamount of dicarboxylic acid components in the copolyester, wherein thecopolyester is a random or alternating copolyester, and wherein afterdeformation by subjecting and maintaining the film to 7% strain, thestress within the film immediately after deformation dissipates to aresidual value of less than 60% of the original value within 10 minutesof said deformation, for the purpose of providing conformability to amedium for signage, advertising, graphic art and other displayapplications or as an overlaminate for such media.

[0047] According to a further aspect of the present invention there isprovided a process for the production of a conformable orientedpolymeric film which comprises melt-extruding a layer of copolyesterderived from

[0048] (i) one or more diol(s);

[0049] (ii) an aromatic dicarboxylic acid; and

[0050] (iii) one or more aliphatic dicarboxylic acid(s) of the generalformula C_(n)H_(2n)(COOH)₂ wherein n is 2 to 8,

[0051] wherein the aromatic dicarboxylic acid is present in thecopolyester in an amount of from about 60 to about 85 mole % based onthe total amount of dicarboxylic acid components in the copolyester andwherein the copolyester is a random or alternating copolyester, andfurther comprising the step of stretching the extrudate in at least onedirection.

[0052] The film may conveniently contain any of the additivesconventionally employed in the manufacture of polymeric films. Thus,agents such as cross-linking agents, dyes, pigments, voiding agents,lubricants, anti-oxidants, radical scavengers, UV absorbers, thermalstabilisers, flame retardants and inhibitors, anti-blocking agents,surface active agents, slip aids, optical brighteners, gloss improvers,prodegradents, viscosity modifiers and dispersion stabilisers may beincorporated as appropriate. In particular, the film may comprise aparticulate filler which can improve handling and windability duringmanufacture. The particulate filler may, for example, be a particulateinorganic filler or an incompatible resin filler or a mixture of two ormore such fillers.

[0053] By an “incompatible resin” is meant a resin which either does notmelt, or which is substantially immiscible with the polymer, at thehighest temperature encountered during extrusion and fabrication of thefilm. The presence of an incompatible resin usually results in a voidedlayer, by which is meant that the layer comprises a cellular structurecontaining at least a proportion of discrete, closed cells. Suitableincompatible resins include polyamides and olefin polymers, particularlya homo- or co-polymer of a mono-alpha-olefin containing up to 6 carbonatoms in its molecule. Preferred materials include a low or high densityolefin homopolymer, particularly polyethylene, polypropylene orpoly-4-methylpentene-1, an olefin copolymer, particularly anethylene-propylene copolymer, or a mixture of two or more thereof.Random, block or graft copolymers may be employed.

[0054] Particulate inorganic fillers include conventional inorganicfillers, and particularly metal or metalloid oxides, such as alumina,silica (especially precipitated or diatomaceous silica and silica gels)and titania, calcined china clay and alkaline metal salts, such as thecarbonates and sulphates of calcium and barium. The particulateinorganic fillers may be of the voiding or non-voiding type. Suitableparticulate inorganic fillers may be homogeneous and consist essentiallyof a single filler material or compound, such as titanium dioxide orbarium sulphate alone. Alternatively, at least a proportion of thefiller may be heterogeneous, the primary filler material beingassociated with an additional modifying component. For example, theprimary filler particle may be treated with a surface modifier, such asa pigment, soap, surfactant coupling agent or other modifier to promoteor alter the degree to which the filler is compatible with thefilm-forming copolyester.

[0055] Preferred particulate inorganic fillers include titanium dioxideand silica.

[0056] Titanium dioxide particles may be of anatase or rutile crystalform. The titanium dioxide particles preferably comprise a major portionof rutile, more preferably at least 60% by weight, particularly at least80%, and especially approximately 100% by weight of rutile. Theparticles can be prepared by standard procedures, such as the chlorideprocess or the sulphate process. The titanium dioxide particles may becoated, preferably with inorganic oxides such as aluminium, silicon,zinc, magnesium or mixtures thereof. Preferably the coating additionallycomprises organic compound(s), such as fatty acids and preferablyalkanols, suitably having from 8 to 30, preferably from 12 to 24 carbonatoms. Polydiorganosiloxanes or polyorganohydrogensiloxanes, such aspolydimethylsiloxane or polymethylhydrogensiloxane are suitable organiccompounds. The coating is suitably applied to the titanium dioxideparticles in aqueous suspension. The inorganic oxides are precipitatedin aqueous suspension from water-soluble compounds such as sodiumaluminate, aluminium sulphate, aluminium hydroxide, aluminium nitrate,silicic acid or sodium silicate. The coating layer on the titaniumdioxide particles is preferably in the range from 1 to 12% of inorganicoxides, and preferably in the range from 0.5 to 3% of organic compound,by weight based upon the weight of titanium dioxide.

[0057] The inorganic filler should be finely-divided, and the volumedistributed median particle diameter (equivalent spherical diametercorresponding to 50% of the volume of all the particles, read on thecumulative distribution curve relating volume % to the diameter of theparticles—often referred to as the “D(v,0.5)” value) thereof ispreferably in the range from 0.01 to 5 μm, more preferably 0.05 to 1.5μm, and particularly 0.15 to 1.2 μm.

[0058] The size distribution of the inorganic filler particles is alsoan important parameter, for example the presence of excessively largeparticles can result in the film exhibiting unsightly ‘speckle’, i.e.where the presence of individual filler particles in the film can bediscerned with the naked eye. It is preferred that none of the inorganicfiller particles incorporated into the film should have an actualparticle size exceeding 30 μm. Particles exceeding such a size may beremoved by sieving processes which are known in the art. However,sieving operations are not always totally successful in eliminating allparticles greater than a chosen size. In practice, therefore, the sizeof 99.9% by number of the inorganic filler particles should not exceed30 μm, preferably should not exceed 20 μm, and more preferably shouldnot exceed 15 μm. Preferably at least 90%, more preferably at least 95%by volume of the inorganic filler particles are within the range of thevolume distributed median particle diameter ±0.8 μm, and particularly±0.5 μm.

[0059] Particle size of the filler particles may be measured by electronmicroscope, coulter counter, sedimentation analysis and static ordynamic light scattering. Techniques based on laser light diffractionare preferred. The median particle size may be determined by plotting acumulative distribution curve representing the percentage of particlevolume below chosen particle sizes and measuring the 50th percentile.

[0060] The components of the composition of a layer may be mixedtogether in a conventional manner. For example, by mixing with themonomeric reactants from which the film-forming copolyester is derived,or the components may be mixed with the copolyester by tumble or dryblending or by compounding in an extruder, followed by cooling and,usually, comminution into granules or chips. Masterbatching technologymay also be employed.

[0061] In a preferred embodiment, the film of the present invention isoptically clear, preferably having a % of scattered visible light (haze)of <10%, preferably <6%, more preferably <3.5% and particularly <2%,measured according to the standard ASTM D 1003. In this embodiment,filler is typically present in only small amounts, generally notexceeding 0.5% and preferably less than 0.2% by weight of a given layer.

[0062] In an alternative embodiment, the film is opaque and highlyfilled, preferably exhibiting a Transmission Optical Density (TOD)(Sakura Densitometer; type PDA 65; transmission mode) in the range from0.1 to 2.0, more preferably 0.2 to 1.5, more preferably from 0.25 to1.25, more preferably from 0.35 to 0.75 and particularly 0.45 to 0.65.The film is conveniently rendered opaque by incorporation into thepolymer blend of an effective amount of an opacifying agent. Suitableopacifying agents include an incompatible resin filler, a particulateinorganic filler or a mixture of two or more such fillers, ashereinbefore described. The amount of filler present in a given layer ispreferably in the range from 1% to 30%, more preferably 3% to 20%,particularly 4% to 15%, and especially 5% to 10% by weight, based on theweight of the layer polymer.

[0063] The surface of an opaque film preferably exhibits a whitenessindex, measured as herein described, in the range from 60 to 120, morepreferably 80 to 110, particularly 90 to 105, and especially 95 to 100units.

[0064] The surface of the film may have thereon one or more furtherpolymeric layers or coating materials. Any coating is preferablyperformed “in-line”.

[0065] In one embodiment, the additional coating may comprise a “slipcoating” in order to improve the handling and windability of the film. Asuitable slip coating may be, for instance a discontinuous layer of anacrylic and/or methacrylic polymeric resin optionally further comprise across-linking agent, such as described in EP-A-0408197, the disclosureof which is incorporated herein by reference. An alternative slipcoating may comprise a potassium silicate coating, for instance asdisclosed in U.S. Pat. Nos. 5,925,428 and 5,882,798, the disclosures ofwhich are incorporated herein by reference.

[0066] In a further embodiment, the film has on one surface thereof ahardcoat or scratch resistant layer and optionally a primer layer (suchas that disclosed in U.S. Pat. No. 3,443,950) between the film and thehardcoat. The hardcoat layer provides a degree of mechanical protectionto the film, as judged for example by the Taber abraser test (ASTMMethod D-1044) with percent haze on the samples determined by ASTMMethod D-1003. Suitable hardcoat layers are disclosed in, for instance,U.S. Pat. No. 3,708,225, U.S. Pat. No. 4,177,315, U.S. Pat. No.4,309,319, U.S. Pat. No. 4,436,851 and U.S. Pat. No. 4,455,205, thedisclosures of which are incorporated herein by reference.

[0067] In a further embodiment, the film has on one surface thereof anadhesive layer. The adhesive layer may comprise any suitable adhesive,for instance a pressure-sensitive adhesive, and is preferably one whichallows the film to be positioned and readily repositioned on a surfaceand finally removed from a surface without leaving an adhesive residue.A suitable repositionable adhesive is described in U.S. Pat. No.4,882,211 and identified as Note Stix®. Further suitable adhesives aredescribed in U.S. Pat. No. 5,198,301 and the art referred to therein,particularly U.S. Pat. No. 3,691,140 and U.S. Pat. No. 4,735,837 whichdescribe repositionable adhesives. The disclosures of these documentsrelating to adhesive compositions is incorporated herein by reference.Permanent adhesives may also be used. Preferably, the film alsocomprises a release layer, which is removably adhered to the adhesivelayer, as are well known in the art.

[0068] In a further embodiment, the film has on one surface thereof aprintable or ink-receiving layer, and optionally a primer layer (such asthat disclosed in EP-0680409, EP-0429179, EP-0408197, EP-0576179 orWO-97/37849, the disclosures of which are incorporated herein byreference) between the film and the printable or ink-receiving layer inorder to increase adhesion. Suitable printable or ink-receiving layersare disclosed in, for instance, EP-0696516, U.S. Pat. No. 5,888,635,U.S. Pat. No. 5,663,030, EP-0289162, EP-0349141, EP-0111819 andEP-0680409, the disclosures of which are incorporated herein byreference.

[0069] The exposed surface of the film may, if desired, be subjected toa chemical or physical surface-modifying treatment to improve the bondbetween that surface and a subsequently applied layer. A preferredtreatment, because of its simplicity and effectiveness, is to subjectthe exposed surface of the film to a high voltage electrical stressaccompanied by corona discharge. Alternatively, the film may bepretreated with an agent known in the art to have a solvent or swellingaction on the substrate polymer. Examples of such agents, which areparticularly suitable for the treatment of a polyester substrate,include a halogenated phenol dissolved in a common organic solvent e.g.a solution of p-chloro-m-cresol, 2,4-dichlorophenol, 2,4,5- or2,4,6-trichlorophenol or 4-chlororesorcinol in acetone or methanol.

[0070] The preferred treatment by corona discharge may be effected inair at atmospheric pressure with conventional equipment using a highfrequency, high voltage generator, preferably having a power output offrom 1 to 20 kw at a potential of 1 to 100 kv. Discharge isconventionally accomplished by passing the film over a dielectricsupport roller at the discharge station at a linear speed preferably of1.0 to 500 m per minute. The discharge electrodes may be positioned 0.1to 10.0 mm from the moving film surface.

[0071] The invention is illustrated by FIGS. 1 to 4 which show thestress relaxation properties of the films as a graph of stress (kgf/mm²)against time after deformation, or as a graph of the stress reduction (%stress relative to the stress immediately after deformation) againsttime after deformation.

[0072]FIG. 1 shows the stress relaxation of Example 6, Example 9 andExample 12 illustrating the effects of the draw ratio on stressrelaxation in the machine direction. Examples 6, 9 and 12 were preparedusing draw ratios of 2.5, 3.5 and 4.5 respectively in both the machineand transverse directions. The data show that as the draw ratioincreases the initial and residual stress in the machine direction ofthe film after deformation is higher.

[0073]FIG. 2 shows the stress relaxation in the machine direction ofExamples 6, 9 and 12 as a graph of stress at time t as a % of stress att=0 plotted against time t. The data show that low draw ratios willpromote a greater degree of stress relaxation.

[0074]FIGS. 3 and 4 show the data corresponding to FIGS. 1 and 2respectively in respect of the transverse direction of the films ofExamples 6, 9 and 12. The variation in the data in FIG. 3 corresponds tothat of FIG. 1 in that the initial and residual stress in the transversedirection of the film is higher as draw ratio increases. However, FIG. 4shows that despite the different internal stress present in each filmunder test, the degree of stress relaxation with time in the transversedirection is similar for all examples.

[0075] The following test methods may be used to determine certainproperties of the polymeric film:

[0076] (i) Wide angle haze is measured using a Hazegard System XL-211,according to ASTM D 1003-61.

[0077] (ii) Whiteness index is measured using a Colorgard System 2000,Model/45 (manufactured by Pacific Scientific) based on the principlesdescribed in ASTM D313.

[0078] (iii) Stress and stress relaxation is measured by stretching thefilm sample by 7% at 21° C. (room temperature) in an InstronExtensometer, model 4464, and holding at that elongation for theduration of the test. The Instron instrument also measures and recordsat intervals over the subsequent 10 minutes, the load applied tomaintain the sample at this elongation. From those readings, the stresswithin the film is calculated and a stress relaxation graph obtained byplotting the measured stress versus time.

[0079] (iv) Unless otherwise specified, shrinkage was measured using aPerkin-Elmer TMA-7 instrument. Samples of film were heated in air underminimal load, to 100° C. and allowed to shrink freely at thattemperature for 60 minutes. Final shrinkage was measured after coolingto room temperature and expressed as a percentage of the original lengthof specimen.

[0080] For the purposes of comparison with PVC film, a further,alternative method was employed in which a sample of film with initialdimensions 100 mm×100 mm was heated unrestrained in an oven at 100° C.(see Table 2 and Example 9 and the p-PVC Comparative Example). Shrinkagewas again expressed as a percentage of the original length of specimenafter cooling, with reference to the MD and TD axes of the film.

[0081] (v) The tear properties of the films of the present invention arecharacterised using two parameters, namely the “maximum load” and the“tear toughness”, measured as described below.

[0082] The maximum load is a measure of the force required to initiatetearing of the film, i.e. the load at the onset of tearing, and ismeasured in accordance with ASTM D1004-94A (Graves Tear Test). Thisparameter is referred to in ASTM D1004-94A as the initialtear-resistance, and is expressed in Newtons or kilograms-force. Themaximum load values reported herein have been normalised to a referencethickness of 50 μm.

[0083] The tear toughness is measured as the Graves Area of the film, asdescribed in EP-A-0592284. The Graves Area is obtained by mathematicallyintegrating the area beneath the curve in a graphical plot of the stressversus the strain for a film subjected to the Graves Tear Test (ASTMD1004-94A), i.e. during a test in which a film sample specificallyshaped for a Graves Tear Test is clamped between opposed jaws that aremoved apart at a constant rate to concentrate the tearing stresses in asmall area. The stress is defined as the recorded load divided by theinitial cross-sectional area of the film opposite the notch feature ofthe test sample. The strain is defined as the ratio of the change in thedistance between the jaws (Δl) that occurs during the test, to theinitial separation of the jaws (l), i.e. strain is Δl/l. Thus, teartoughness may be regarded as a measure of the total energy required tocause the film to fail, i.e. the ability of the film to absorb energybefore failure. It will be understood that film with a relatively largetear toughness value will require a larger amount of total energy tocause failure, compared to film with a relatively small tear toughnessvalue. The tear toughness may vary depending on whether the test isconducted in the machine or the transverse direction of the film.Preferably, films according to the invention demonstrate a teartoughness in one direction of the film equal to at least 0.3 kg/mm²,preferably at least 0.6 kg/mm², and more preferably at least 0.9 kg/mm².

[0084] The invention is further illustrated by the following examples.It will be appreciated that the examples are for illustrative purposesonly and are not intended to limit the invention as described above.Modification of detail may be made without departing from the scope ofthe invention.

EXAMPLES

[0085] A series of copolyester compositions were synthesised usingconventional techniques, as described herein, and films preparedtherefrom. The general method of film preparation is set out below. Thespecific details of the copolyester films and the stress relaxationproperties thereof are shown in Table 1. The value for Tg in Table 1 isthat of the copolyester itself rather than that of the film. Thecopolyesters are all based on poly(ethylene terephthalate) containingadipic acid (AA) as comonomer. The amount of adipic acid in Table 1 isgiven as a percentage of the total dicarboxylic acid content of thecopolyester (i.e. the amount of terephthalic acid of the normal PETrecipe which is substituted by adipic acid). Some of the copolyestersalso contain a second glycol (neopentyl glycol (nPG); orcyclohexanedimethanol (CHDM)) as comonomer. The amount of the secondglycol is given as a percentage of the total glycol content of thecopolyester.

[0086] The polymer composition was extruded and cast onto a cooledrotating drum and, in most instances, subjected to sequential stretchingoperations, as described herein. This involved first stretching the filmin the direction of extrusion at a temperature of 45° C. and theneffecting a second stretching step in the sideways direction, also at atemperature of 45° C.

[0087] The films prepared in this way are those identified by the “SEQ”reference in Table 1. In some cases, the forward and sideways draws wereperformed simultaneously and the films prepared in this way are thoseidentified by the “SIM” reference in Table 1. The biaxially stretchedfilm was heat-set under full restraint by conventional means. The drawratios, i.e. the ratio of the stretched dimension to the originaldimension in each direction, and the heat-set temperatures are given inTable 1. SEQ means that the forward and sideways draws were performed insequence; SIM means that both draws were performed simultaneously.

[0088] The films were characterised using the stress relaxation testdescribed herein and the results are shown in Table 1. The stressrelaxation values refer to the stress remaining in a sample of filmwhich is subject to and maintained at a fixed strain of 7% after setperiods of time (3 or 10 minutes) at room temperature. Measurements inboth the machine direction (MD) and the transverse direction (TD) weretaken and the % residual stress calculated.

[0089] The results are also displayed graphically in FIGS. 1 to 4, asdescribed above.

[0090] The data in Table 1 shows that the films of the present inventionexhibit good stress relaxation, with as little as 38% residual stress insome films and absolute values of around 0.6 kg/mm² after 10 minutes.TABLE 1 Stress Relaxation Max. stress Draw Heat (kg/mm²): Ratios Set 0minutes Stress after 3 minutes (kg/mm²) Stress after 10 minutes (kg/mm²)Ex. Composition Tg (C) MD × SD (C.) MD TD MD MD (%) TD TD (%) MD MD (%)TD TD (%) 1 29.4% AA 29° 2.6 × 3.1 100° 2.96 3.85 1.42 48 1.85 48 1.2342 1.6 42 2 SEQ 120° 2.73 2.59 1.25 46 1.14 44 1.09 40 0.99 38 3 160°1.32 1.21 0.553 42 0.557 46 0.48 36 0.48 40 4 29.4% AA 29° 2.5 × 2.5100° 1.97 2.24 0.906 46 1.01 45 0.78 40 0.87 39 5 SEQ 120° 1.34 1.550.632 47 0.697 45 0.55 41 0.6 39 6 160° 0.89 0.89 0.4 45 0.42 47 0.35 390.36 41 7 29.4% AA 29° 3.5 × 3.5 100° 3.54 4.36 1.7 48 2.27 52 1.47 421.96 45 8 SEQ 120° 2.85 3.21 1.4 49 1.54 48 1.21 42 1.33 42 9 160° 1.461.52 0.64 44 0.67 44 0.55 38 0.58 38 10 29.4% AA 29° 4.5 × 4.5 100° 2.012.66 1 50 1.41 53 0.87 43 1.22 46 11 SEQ 120° 2.26 2.62 1.13 50 1.39 530.98 43 1.2 46 12 160° 1.9 1.67 0.99 52 0.8 48 0.85 45 0.7 42 13 29.4%AA 29° 4.5 × 4.5 100° 3.76 4.04 1.95 52 2.02 50 1.69 45 1.75 43 14 SIM120° 3.95 3.81 1.98 50 1.83 48 1.71 43 1.58 42 15 28% AA 31° 3.5 × 3.5100° 4.441 1.961 44 1.628 37 16 SEQ 120° 3.206 1.383 43 1.278 40 17 160°2.077 0.782 37 0.648 31 18 24% AA, 40° 3.5 × 3.5 100° 7.029 3.23 462.994 43 19  7% nPG SEQ 120° 8.238 2.734 33 2.315 28 20 160° 4.564 1.34930 0.974 21 21 24% AA, 41° 3.5 × 3.5 100° 7.103 3.061 43 2.721 38 22 10%CHDM SEQ 120° 6.287 2.727 43 2.401 38 23 160° 4.504 1.511 34 1.174 26

[0091] The films were also characterised by measuring their shrinkageusing the test methods described herein. Example 9 and the P-PVCComparative Example were analysed using the alternative test methoddescribed herein, i.e. as square specimens (100 mm×100 mm) in an airoven at 100° C. The results are shown in Table 2. TABLE 2 % ShrinkageExample MD TD Average 4 2.1 −1.6 0.25 5 0.8 0.2 0.5 6 1.2 0 0.8 10  0.5−0.6 −0.05 11  0.8 1.4 1.1 12  2.4 1.6 2.1 9 2.2 1.8 2.0 p-PVCComparative 6.0 2.0 4.0 Example

[0092] A negative value for the % shrinkage indicates an expansion inthe test used.

[0093] The data show that in all cases the copolyester films of thepresent invention show better dimensional stability than the plasticisedPVC Comparative Example, and when manufactured under optimum conditionsshow shrinkage properties which are significantly superior.

[0094] The films were also characterised by measuring their tearresistance using the test method described herein. The results are shownin Table 3. TABLE 3 Tear Resistance in MD by Graves Method Maximum LoadTear Toughness Example (50μ film) (kgf) (kg/mm²)  4 1.436 1.779  5 1.6802.194  7 1.206 1.272  8 1.588 1.388  9 1.106 1.366 10 2.250 0.938 111.778 0.927 15 2.227 0.669 17 1.191 0.722 18 2.058 0.756 20 1.335 0.98921 2.191 0.773 23 1.288 0.843 p-PVC Comparative 0.766 0.536 Example

[0095] The date show that in all cases the films of the presentinvention exhibit better tear resistance than the plasticised PVCComparative Example.

1. A conformable oriented polymeric film comprising a layer ofcopolyester derived from: (1) one or more diol(s); (2) an aromaticdicarboxylic acid; and (3) one or more aliphatic dicarboxylic acid(s) ofthe general formula C_(n)H_(2n)(COOH)₂ wherein n is 2 to 8, wherein thearomatic dicarboxylic acid is present in the copolyester in an amount offrom 60 to 85 mole % based on the total amount of dicarboxylic acidcomponents in the copolyester, wherein the copolyester is a random oralternating copolyester, and wherein after deformation by subjecting andmaintaining the film to 7% strain, the stress within the filmimmediately after deformation dissipates to a residual value of lessthan 60% of the original value within 10 minutes of said deformation. 2.A film according to claim 1 which is biaxially oriented.
 3. A filmaccording to claim 1 wherein the copolyester is derived from only onediol.
 4. A film according to claim 1 wherein the copolyester is derivedfrom ethylene glycol.
 5. A film according to claim 1 wherein saidaromatic dicarboxylic acid is terephthalic acid.
 6. A film according toclaim 1 wherein the copolyester is derived from only one aliphaticdicarboxylic acid.
 7. A film according to claim 1 wherein said aliphaticdicarboxylic acid is succinic acid, glutaric acid, adipic acid, pimelicacid, suberic acid, azeleic acid or sebacic acid.
 8. A film according toclaim 1 wherein said aliphatic dicarboxylic acid is adipic acid.
 9. Afilm according to claim 1 wherein said aromatic dicarboxylic acid ispresent in the copolyester in an amount of about 65 to about 75 mole %based on the total amount of dicarboxylic acid components in thecopolyester.
 10. A film according to claim 1 wherein said copolyester isa random copolyester.
 11. A film according to claim 1 wherein the glasstransition temperature of said copolyester is below about 35° C.
 12. Afilm according to claim 1 wherein, after deformation by subjecting andmaintaining the film to 7% strain, the stress within the filmimmediately after deformation dissipates to a residual value of lessthan 40% of the original value within 10 minutes of said deformation.13. A film according to claim 1 wherein, after deformation by subjectingand maintaining the film to 7% strain, the initial stress within thefilm is less than 4.5 kg/mm².
 14. A film according to claim 1 wherein,after deformation by subjecting and maintaining the film to 7% strain,the residual stress within 10 minutes of the said deformation is lessthan 1.7 kg/mm².
 15. A film according to claim 1 wherein the shrinkageafter treatment in air at 100° C. for 60 minutes is less than 4% in boththe machine and transverse dimensions.
 16. Use of a conformable orientedpolymeric film comprising a layer of copolyester derived from: (i) oneor more diol(s); (ii) an aromatic dicarboxylic acid; and (iii) one ormore aliphatic dicarboxylic acid(s) of the general formulaC_(n)H_(2n)(COOH)₂ wherein n is 2 to 8, wherein the aromaticdicarboxylic acid is present in the copolyester in an amount of from 60to 85 mole % based on the total amount of dicarboxylic acid componentsin the copolyester, wherein the copolyester is a random or alternatingcopolyester, and wherein after deformation by subjecting and maintainingthe film to 7% strain, the stress within the film immediately afterdeformation dissipates to a residual value of less than 60% of theoriginal value within 10 minutes of said deformation, as or in themanufacture of a conformable medium for signage, advertising, graphicart and other display applications or as an overlaminate for such media.17. A process for the production of a conformable oriented polymericfilm which comprises melt-extruding a layer of copolyester derived from:(i) one or more diol(s); (ii) an aromatic dicarboxylic acid; and (iii)one or more aliphatic dicarboxylic acid(s) of the general formulaC_(n)H_(2n)(COOH)₂ wherein n is 2 to 8, wherein the aromaticdicarboxylic acid is present in the copolyester in an amount of from 60to 85% mole % based on the total amount of dicarboxylic acid componentsin the copolyester and wherein the copolyester is a random oralternating copolyester, and further comprising the step of stretchingthe extrudate in at least one direction.